Cherry-pick #19567 to 7.x: [Elastic Agent] Properly stop subprocess when receiving SIGTERM

[blakerouse]

Cherry-pick of PR #19567 to 7.x branch. Original message:

What does this PR do?

Fixes issue where if Elastic Agent is stopped the spawned subprocesses are not stopped. This ensures that on SIGTERM to Elastic Agent that it properly shutdowns the subprocesses, by sending the ExpectedState_Stopping over GRPC and waiting for the process to stop (kill it after 30 seconds, if it doesn't stop).

On Linux in the case that Elastic Agent is kill -9 all its spawned subprocesses will also be kill -9, without having to worry that any children processes are still handing around. Something that Windows or Mac does not support.

Why is it important?

So subprocess are not orphaned when Elastic Agent is stopped or killed.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• [ ] I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

• Closes [Elastic Agent] when Agent is stopped, Metricbeat & Filebeat are not stopped #19522

Logs

2020-07-01T15:24:09-04:00 INFO  app.go:160      Signaling application to stop because of shutdown: metricbeat(metricbeat--8.0.0-SNAPSHOT--36643631373035623733363936343635)
2020-07-01T15:24:09-04:00 INFO  reporter.go:51  2020-07-01T15:24:09-04:00: type: 'STATE': sub_type: 'STOPPED' message: Application: filebeat--8.0.0-SNAPSHOT--36643631373035623733363936343635[a2c055bc-831f-48dd-9204-075f88cd5760]: State changed to STOPPED: Stopped
2020-07-01T15:24:09-04:00 INFO  app.go:160      Signaling application to stop because of shutdown: metricbeat(metricbeat--8.0.0-SNAPSHOT)
2020-07-01T15:24:09-04:00 INFO  reporter.go:51  2020-07-01T15:24:09-04:00: type: 'STATE': sub_type: 'STOPPED' message: Application: metricbeat--8.0.0-SNAPSHOT--36643631373035623733363936343635[a2c055bc-831f-48dd-9204-075f88cd5760]: State changed to STOPPED: Stopped
2020-07-01T15:24:12-04:00 INFO  reporter.go:51  2020-07-01T15:24:12-04:00: type: 'STATE': sub_type: 'STOPPED' message: Application: metricbeat--8.0.0-SNAPSHOT[a2c055bc-831f-48dd-9204-075f88cd5760]: State changed to STOPPED: Stopped

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: [Pull request #19683 opened]

• Start Time: 2020-07-06T18:05:42.522+0000

• Duration: 32 min 49 sec

[michalpristas]

LGTM